1. Field of the Invention
The present invention relates to an image processing device, an image processing method, and an imaging apparatus, and, more particularly to an image processing device and an image processing method for processing an output signal of a solid-state imaging device having a filter of predetermined color coding (a color filter), and an imaging apparatus that uses the image processing device or the image processing method.
2. Description of the Related Art
In recent years, image qualities of imaging apparatuses such as a digital still camera and a video camera have been improved and imaging apparatuses of various specifications have been proposed. As a representative example, there is known a 3CCD imaging apparatus that obtains high-resolution RGB signals by arranging primary color filters of red (R), green (G), and blue (B) in the same spatial phase with respect to three solid-state imaging devices and realizes a high image quality. However, the 3CCD imaging apparatus needs to use the three solid-state imaging devices and use a prism for dissolving incident light into colors of R, G, and B. This makes it difficult to realize a reduction in size and a reduction in cost.
On the other hand, there is also known a 1CCD imaging apparatus that realizes a reduction in size and a reduction in cost by appropriately arranging color filters of R, G, and B in pixel units in one solid-state imaging device. In order to obtain high luminance resolution in this 1CCD imaging apparatus, a color arrangement in arranging the color filters of R, G, and B in pixel units in one solid-state imaging device is important. A characteristic of the solid-state imaging device varies depending on a way of the arrangement of the color filters. As a representative color coding (color arrangement), for example, there is known the Bayer arrangement that has been widely used.
(Bayer Arrangement)
As shown in FIG. 24, the Bayer arrangement is color coding in which color filters of R and color filters of G are alternately arranged in odd number rows and color filters of G and color filters of B are alternately arranged in even number rows. The Bayer arrangement has a characteristic that resolution of G is higher than resolution of R and B because a larger number of color filters of G are arranged compared with the color filters of R and B.
In general, a luminance signal Y is generated according to Expression (1) below.Y=0.6G+0.3R+0.1B  (1)
As it is evident from Expression (1), G is a main component in generating the luminance signal. Human being has a visual sensitivity characteristic that resolution for luminance is high and resolution for color is low. It can be said that the Bayer arrangement is a color arrangement that successfully utilizes the visual sensitivity characteristic of human being.
However, the color coding of the Bayer arrangement is not always optimum for human being. This is because human eyes have a characteristic that it is possible to recognize a high frequency for luminance but it is difficult to recognize a high frequency for color.
In signal processing of an imaging apparatus, generation of a luminance signal or a color difference signal from RGB signals having spatially different phases is a cause of generation of a false color signal. Thus, luminance signal processing and color difference signal processing have to be performed after generating RGB signals having the same spatial phases. Therefore, it is important to generate the RGB signals in the same spatial phase from the RGB signals having the spatially different phases according to the interpolation technique. If the interpolation is not optimum, for example, an image having high resolution is not obtained or a false color signal is generated.
In order to perform the interpolation processing, in the past, amounts of change in eight pixels in total near a pixel of attention, that is, pixels above and below, on the right and the left, and at the upper right, the lower right, the upper left, and the lower left of the pixel of attention are calculated. The amounts of change calculated are weighted to calculate correlation values. Interpolation coefficients are determined on the basis of the correlation values calculated. Interpolation data are multiplied by the interpolation coefficients, respectively, and, then, the interpolation data multiplied by the interpolation coefficients are added up. (See, for example, JP-A-11-177994).
However, in the related art described above, the interpolation processing is performed using correlation values calculated by weighting amounts of change in eight pixels in total above and below, on the right and the left, and at the upper right, the lower right, the upper left, and the lower left of a pixel of attention. Thus, a direction of correlation can only be detected in horizontal, vertical, and 45° directions with respect to the pixel of attention. This makes it difficult to perform appropriate interpolation processing.
Therefore, it is desirable to provide an image processing device, an image processing method, and an imaging apparatus that are capable of performing appropriate interpolation processing by judging a direction of correlation in all the directions (360°) with respect to a pixel of attention to be interpolated.